In the past, gyros and accelerometers have been used in a combination, in order to provide inputs to a gyro navigation system. These inputs have been used by the gyro navigation system in order to provide a geographic position of a vehicle on which the system is placed.
However, with the passage of time, sensor axes of the gyros becomes less accurate, due to drift of spin axes of the rotors of the gyros. The provided geographic position of the vehicle becomes less accurate with time, due to such drift of the spin axes of the rotors of the gyros.
The passive position fix system of the present invention uses gyros and accelerometers of a gyro navigation system on a vehicle to find a gyro position of a vehicle on which the passive position fix system has been placed. The passive position fix system compares stored a gravity gradient and gravity anomaly map-set, for each of several geographic positions in a region around the gyro position of the vehicle, with measurements from gravity gradient sensors and a gravity anomaly sensor. The passive position fix system finds a best estimate of the position difference between the gyro position and the geographic position of the vehicle. The gyro navigation system is updated with this best estimate of position difference. The passive position fix system thus provides an updated gyro position of the vehicle.
The gyro navigation system, gravity gradient and gravity anomaly map-sets, and a gravity gradient and gravity anomaly sensor system are central parts of the passive position fix system.
The gravity gradient and gravity anomaly sensor system and the gravity gradient and gravity anomaly map-sets are used in a map matching technique in the passive position fix system. The gravity gradient and gravity anomaly sensor system and the gravity gradient and gravity anomaly map-sets can be used to periodically update the gyro position provided by the gyro navigation system.
The gravity gradient and gravity anomaly sensor system and gravity gradient and gravity anomaly map-sets could be used independently to provide geographic locations of a vehicle.
The passive position fix system has a gravity gradient and gravity anomaly sensor system. The gravity gradient and gravity anomaly sensor system firstly measures six gravity related parameters. The six measured parameters are five gravity gradients and one gravity anomaly.
These six measured parameters are then successively compared to each of several stored map-sets for geographic positions in the region of the gyro position as provided by the gyro navigation system. A stored map-set is a set of stored map parameters for a corresponding geographic position. A set of stored map parameters consists of five predetermined gravity gradients and one predetermined gravity anomaly of a corresponding geographic position.
Information derived from comparisons of the map-sets with the six measured gravity related parameters is used by a real-time filtering algorithm to compute a displacement vector between the gyro position as provided by the gyro navigation system and the geographic position whose map-set provides a match. The displacement vector is used to update the gyro position of the gyro navigation system of the passive position fix system.
An objective of the passive position fix system (PPFS) is to allow an operator on a vehicle to accurately determine the geographic position of the vehicle, in real-time, without exposure or radiation of energy, in a completely passive manner.
The objectives of the PPFS are:
a) passivity
b) accuracy
c) manual/automatic fix site selection and course guidance from site to site